Device and method for producing silicon blocks

ABSTRACT

Device for producing silicon blocks for photovoltaic applications, comprising a container for receiving a silicon melt with a base wall and at least one side wall, means for reducing the diffusion of impurities from at least one of the walls of the container into the silicon melt, wherein the means for reducing the diffusion of impurities comprise at least one covering element for the at least partial covering of at least one of the walls of the container.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application,Serial No. 10 2011 082 628.9, filed Sep. 13, 2011, pursuant to 35 U.S.C.119(a)-(d), the content of which is incorporated herein by reference inits entirety as if fully set forth herein.

FIELD OF THE INVENTION

The invention relates to a device for producing silicon blocks. Inaddition, the invention relates to a method for producing silicon blocksand to silicon blocks.

BACKGROUND OF THE INVENTION

The production of large-volume semiconductor bodies, in particularsilicon blocks, is of fundamental importance for the production ofsilicon solar cells. Melting crucibles, in which a silicon melt issolidified, are generally used to produce silicon blocks. A device and amethod for producing silicon blocks is known, for example, from DE 102005 013 410 A1.

There is continuously a need to further develop devices and methods forproducing silicon blocks. This object is achieved by a device forproducing silicon blocks for photovoltaic applications, comprising acontainer for receiving a silicon melt with a base wall and at least oneside wall, and comprising means for reducing the diffusion of impuritiesfrom at least one of the walls of the container into the silicon meltand the silicon block, wherein the means for reducing the diffusion ofimpurities comprise at least one covering element to at least partiallycover at least one of the walls of the container. This object is furtherachieved by a method for producing silicon blocks, comprising the stepsof providing a container to receive a silicon melt comprising a basewall and at least one side wall, providing and arranging a means forseparating the silicon melt in the container from the base wall thereof,providing a silicon melt in the container, and solidifying the siliconmelt.

SUMMARY OF THE INVENTION

A core of the invention is to arrange a diffusion barrier in a cruciblefor receiving a silicon melt. The diffusion barrier is used to reducethe diffusion of impurities from the crucible into the melt and into thecrystal. The achievable yield is increased by this.

An insertion or covering element, which is configured as an insertionplate or insertion foil and is inserted into the crucible, is preferablyprovided as the diffusion barrier. This allows a particularly simplearrangement of the diffusion barrier in the crucible. The rigidinsertion plate and the flexible insertion foil are collectively calledthe covering element.

The insertion plate may have a one-piece configuration. It may alsocomprise a plurality of part plates. The term “insertion plate” is alsotaken to means the multi-part configuration below. The same applies tothe insertion foil.

The covering element, in particular the insertion plate, is adapted tothe dimensions of the container. It is preferably configured in such away that the base wall and/or at least one of the side walls, inparticular all the side walls of the crucible, can be covered by it asfar as possible completely. The base wall and/or the respective sidewall(s) of the crucible can be in particular at least 90%, in particularat least 95%, in particular at least 99%, in particular 99.9%,preferably completely, covered by the insertion plate, The more completethe covering of the walls of the crucible by means of the insertionplate, the more effectively is a diffusion of impurities from thecrucible into the melt and/or the crystal prevented. The insertion plateleads to a spatial separation of the silicon melt or the silicon crystalfrom the base wall and/or the side walls of the crucible.

The covering element is preferably made of a material, which has amelting point, which is above the melting point of silicon. As a result,a melting of the diffusion barrier in the silicon melt is prevented bythis.

The covering element is preferably made from a material having a smallerdiffusion constant of impurities (such as, for example, of transitionmetals) in relation to silicon. The diffusion content of the material ofthe covering element in relation to silicon at temperatures in the rangefrom 20° C. to 1500° C., in particular in the range from 800° C. to1412° C., is smaller than the diffusion constants of one of the elementsselected from the group of the above-mentioned impurities, in particularof iron, in relation to silicon. It is, in particular, at most 0.5times, in particular at most 0.3 times, in particular at most 0.2 timesas large as the diffusion constant of one of the transition metalstitanium, vanadium, chromium, manganese, iron, cobalt or nickel inrelation to silicon at the corresponding temperatures. This ensures thatno significant diffusion of metal impurities occurs from the diffusionbarrier into the silicon melt or the silicon crystal. The diffusion ofmetallic components of the diffusion barrier into the silicon melt orthe silicon crystal is, in particular, limited to a boundary layer witha thickness of at most 1 μm, in particular at most 500 nm, in particularat most 300 nm.

The diffusion constant of the diffusion barrier in relation to theabove-mentioned transition metals is preferably smaller than thecorresponding diffusion constant of the SiO₂ melting crucible, inparticular, the diffusion constant of the diffusion barrier is less than10⁻¹¹ m²/s.

In addition to its function as a diffusion barrier, the covering elementcan preferably simultaneously form a nucleus template for thecrystallization of the silicon melt.

The covering element is preferably made of a material, which has a lowerdiffusion constant for transition metals, in particular for iron, thanthe diffusion constant thereof in silicon. The diffusion constant of thematerial of the covering element in relation to transition metals, inparticular in relation to iron, is in particular so low that it isensured that substances of this type do not diffuse through thediffusion barrier when the silicon melt solidifies. As a result, thebarrier effect of the covering element is ensured.

The covering element may, for example, be produced from a refractorymetal, in particular selected from the group of molybdenum (Mo),tungsten (W) and titanium (Ti), or a compound of one or more of thesesubstances. It may be made of quartz, silicon dioxide, silicon carbideor silicon nitride. In general, it has at least one fraction selectedfrom the group of refractory metals, compounds thereof, quartz, silicondioxide, silicon carbide and silicon nitride and other compounds fromthe quaternary system Si—C—O—N. Moreover, the covering element may havea component of aluminum oxide, of multicrystalline Al₂O₃ or ofmonocrystalline sapphire. The covering element may also consistcompletely of one or more of these materials.

In principle, it is also possible to configure the insertion plate froma flat carrier element, which, on at least one side, in particular onboth sides, in particular completely, has a layer of at least one of theabove-mentioned materials.

A wafer, in particular a silicon carbide wafer or silicon dioxide wafer,may also be used, for example, as the insertion plate.

In an advantageous embodiment, the insertion plate is made of amorphoussilicon dioxide. It may, in particular, be produced from molten silicondioxide. It may, for example, be cut from a block of molten andresolidified silicon dioxide.

The diffusion barrier preferably has a density of at least 90% byvolume. Its function as a diffusion barrier is also ensured by this. Inparticular, impurities are effectively prevented from diffusing throughpores of the diffusion barrier into the silicon melt.

In a further advantageous embodiment, the insertion plate is producedfrom aluminum oxide. It has a purity of at least 98%, preferably atleast 99%, preferably at most 99.99%.

The insertion plate preferably has a density of at least 90% by volume,in particular 95% by volume. It is, in particular, closed andnon-porous. This is, in particular, to be taken to mean that theinsertion plate has a closedpore configuration.

In the case of an insertion plate made of aluminum oxide, this may bemonocrystalline. It may, in particular, be made of sapphire.

The insertion plate preferably has a thickness in the range from 0.001mm to 10 mm, in particular in the range from 0.05 mm to 5 mm, inparticular in the range from 0.4 mm to 1 mm. A diffusion of impuritiesfrom the crucible wall into the melt is effectively prevented at thisthickness.

In an advantageous configuration, it may be provided that the insertionplate is provided with a coating of at least one substance from thequaternary system Si—C—O—N. Furthermore, the coating may consist of B—Nmodifications or have modifications of this type, i.e. components ofboron and/or nitrogen and/or compounds thereof It may also have mixturesof Si—C—O—N and B—N. Si₃N₄ or BN are possible, in particular, as thecoating.

The coating may be powdery. It may have temporary organic additives. Asa result, in particular, the sintering of the insertion plate on thecrucible and/or on the ingot can be prevented.

To produce the device, the insertion plate or the insertion foil isarranged in the crucible.

The insertion plate or the insertion foil may, in particular, be placedin the crucible in such a way that it at least partially, in particularas far as possible completely, in particular at least 90%, in particularat least 95%, in particular at least 99%, preferably completely, coversthe base wall. The insertion plate may also be arranged in the cruciblein such a way that it at least partially, in particular as far aspossible completely, in particular at least 50%, in particular at least70%, covers one or optionally more of the side walls. It can be orientedhere in particular parallel or concentrically with respect to the sidewall. It can, in particular, be configured in such a way that itcompletely covers the region of the side wall adjoining the base wall.

It may be provided that the insertion plate is provided with a coatingbefore arranging it in the container. A substance or a substance mixturefrom the quaternary system Si—C—O—N, in particular, may be provided asthe coating. The sintering of the insertion plate on the crucible may beprevented by a coating of this type. It can, in particular, be achievedby this that the insertion plate and/or the crucible can be used severaltimes.

However, it may also be advantageous to provide the insertion plate witha coating after arranging it in the container. The above-mentionedsubstances are in turn provided as the coating. It can, in particular,be achieved by a retrospective coating that the silicon melt in thecontainer is completely without contact with respect to one or more, inparticular all the walls thereof It can, in particular, be achieved thatthe silicon melt does not wet the container and/or the insertion plate.As a result, a contamination of the melt or the ingot, for example withoxygen or aluminum is prevented, in particular. Moreover, the productionprocess can be simplified by a subsequent coating.

With regard to the advantages of the method according to the inventionfor producing silicon blocks, reference is made to the advantages of thedevice according to the invention.

Further objects of the invention consist in improving silicon blocks, inparticular for use in photovoltaic applications. This object is achievedby the means for separating the silicon melt in the container from thebase wall thereof comprising an insertion element, by means of which thebase wall is at least 50% covered, wherein the insertion elementprevents the diffusion of impurities from the base wall of the containerinto the silicon.

A core of the invention consists in reducing, in a silicon block, theextent of the base and/or peripheral region which can generally not beused for photovoltaic applications. By reducing the extent of the edgeregion to values of at most 50 mm, in particular at most 30 mm, inparticular at most 20 mm, in particular at most 10 mm, in particular atmost 5 mm, in particular at most 3 mm, in particular at most 1 mm, theyield of the crystallization process is significantly improved. Inparticular, the extent of the base region in the longitudinal directionof the silicon block can be reduced to the given values here by adiffusion barrier at the base of the crucible. By arranging a diffusionbarrier in the region of the side walls of the crucible, in particularthe extent of the peripheral region in the lateral direction can bereduced to the given values. These measures for reducing the maximumextent of the edge region can preferably be combined with one another.

It was possible to show that the blocks produced according to theinvention have a charge carrier service life averaged laterally over thecore region, which at each height of the core region is at least 2 μs,in particular at least 3 μs, in particular at least 5 μs. The siliconblocks produced according to the invention therefore have a considerableenlarged fraction, which can be further processed for the use ofphotovoltaic applications.

Further advantages and details of the invention emerge from thedescription of embodiments with the aid of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a total device for producing silicon blocks,

FIG. 2 shows a schematic cross-section through a container for receivinga silicon melt with a diffusion barrier according to a first embodiment,

FIG. 3 shows a schematic cross-section through a container for receivinga silicon melt with a diffusion barrier according to a furtherembodiment,

FIG. 4 shows a schematic cross-section through a container for receivinga silicon melt with a diffusion barrier according to a furtherembodiment,

FIG. 5 shows a schematic cross-section through a container for receivinga silicon melt with a diffusion barrier according to a furtherembodiment,

FIG. 6 a shows a service life topogram of a column cut from a siliconblock produced according the invention,

FIG. 6 b shows the laterally averaged service life corresponding to thetopogram according to FIG. 6 a over the column height,

FIGS. 7 and 8 show an exemplary comparison of the charge carrier servicelife on vertical sections of a multicrystalline silicon block producedwithout or with the arrangement of a diffusion barrier in the crucible,and

FIGS. 9 and 10 show views in accordance with FIGS. 7 and 8 ofmonocrystalline silicon blocks, produced with a corresponding nucleustemplate in the crucible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A device 1 for producing silicon blocks 30 according to a firstembodiment will be described below with reference to FIGS. 1 and 2. Thedevice 1 comprises a container 2 for receiving a silicon melt 3 and acovering element configured as an insertion plate 4.

Used as the container 2, is a mould, in particular a reusable mould, toreceive the silicon melt 3, or a crucible, in particular a meltingcrucible, to melt silicon to produce the silicon melt 3.

The container 2 has a base wall 5 and at least one side wall 6. It mayhave a round, in particular a circular cross-section. In this case, theside wall 6 is hollow-cylindrical. The container 2 may also be cuboidal.In this case, it comprises four side walls 6. This possibility isincluded below, in each case, in the term “the side wall 6”.

The container 2 has a diameter in the range from 10 cm to 2 m, inparticular in the range from 15 cm to 100 cm. In the case of a cuboidalcontainer 2, these details correspond to the side lengths of the basewall 5.

The base wall 5 is planar, in other words, it has a uniform thicknessover its entire extent. It may also be provided with a structuring.

The base wall 5 and the side wall 6 delimit an inner space 7, which isopen at one side and is used to receive the silicon melt.

The base wall 5 has a thickness in the range from 0.5 cm to 5 cm, inparticular in the range from 1 cm to 3 cm. The side wall 6 has a wallthickness, which is exactly as large as the wall thickness of the basewall 5. The side wall 6 may also have a smaller wall thickness than thebase wall 5.

The container 2 is preferably made of quartz or ceramic, in particularof a compound of silicon with at least one of the elements oxygen,nitrogen or carbon. The container 2 may, in particular, be made ofsilicon dioxide (SiO₂), silicon nitride (Si₃N₄) or silicon carbide(SiC). The material of the container 2, in particular, has a density inthe range from 75% by volume to 85% by volume.

The device 1 furthermore comprises a support mould 8, which surroundsthe container 2. The support mould is also open on one side. Itcomprises a base plate 9, which is carried by a frame not shown in thefigures.

Furthermore, the device 1 comprises heating elements. Side heatingelements 10, a cover heating element 11 and a base heating element 12can be distinguished here. It is likewise possible to configure thedevice 1 with a single side heating element 10. Accordingly, a pluralityof cover heating elements 11 and/or base heating elements 12 may beprovided. The side heating elements 10 surround the container 2laterally. The cover heating element 11 is arranged above the container2. The base heating element 12 is arranged below the container 2.

Additionally or as an alternative to the heating elements 10, 11, 12,cooling elements may be provided laterally, above and/or below thecontainer 2.

The heating elements 10, 11, 12 and/or the cooling elements arepreferably controllable. The heating elements 10, 11, 12 and the coolingelements together form a temperature control mechanism to control themelting and/or the directed solidification of the silicon in thecontainer 2. Reference is made, for example, to DE 10 2005 013 410 B4for details of the temperature control mechanism.

The container 2 may also be surrounded by a large number of insulationelements 13.

The container 2 may, in particular, be arranged in an outwardly sealedoff crystallization chamber 14. The crystallization chamber 14 has afeed-through 15 for a flushing tube 16. The crystallization chamber 14can be subjected to flushing gas by means of a flushing gas mechanism 17by way of the flushing tube 16. Argon is provided, in particular, as theflushing gas. Alternatively, another inert protective gas may also beused. The atmosphere in the crystallization chamber 14 can, inparticular, be controlled in a targeted manner by means of the flushinggas mechanism 17. The crystallization chamber 14 and the flushing gasmechanism 17 are also components of the device 1.

Further details of the insertion plate 4 being used as the diffusionbarrier will be described below. The insertion plate 4 is mechanicallyrigid. This facilitates the insertion thereof in the container 2. Theinsertion plate 4 is preferably configured in such a way that it can beinserted with a precise fit in the container 2. It is, in particular,configured in such a way that with its help, the base wall 5 of thecontainer 2 can be covered completely as far as possible. The base wall5 of the container 2 can be covered in particular at least 90%, inparticular at least 95%, in particular at least 99%, preferablycompletely, by means of the insertion plate 4. The insertion plate 4 maybe in direct contact with the base wall 5. It may also be in directcontact with the side wall 6. The insertion plate 4 may lead to aspatial separation of the silicon melt 3 and the silicon crystal fromthe base wall 5 and/or the side walls 6 of the container 2. Theinsertion plate 4 may be one-part or multi-part.

A highly pure, coated so-called quartz glass plate is used, for example,as the insertion plate 4. The insertion plate 4 is, in particular, madeof amorphous material. According to the first embodiment, the insertionplate 4 has at least one fraction of silicon dioxide (SiO₂). It has, inparticular, at least one layer of silicon dioxide (SiO₂). It maypreferably be completely produced from silicon dioxide (SiO₂). It may,in particular, be produced from molten amorphous silicon dioxide (SiO₂),for example cut from a block of molten and resolidified silicon dioxide(SiO₂). This leads to a particularly high density and purity of theinsertion plate 4. The insertion plate 4 may, in particular, beclosed-pore.

In principle, other substances, in particular silicon compounds, inparticular compounds of silicon with one or more of the elements oxygen,carbon and nitrogen, and aluminum oxide, are also possible as thematerial of the insertion plate 4. Moreover, the covering element mayconsist of aluminum oxide, of multi-crystalline Al₂O₃ or ofmonocrystalline sapphire.

In general, the material of the insertion plate 4 has a coefficient ofdiffusion for impurities in the container 2, in particular for metals,which is smaller than the coefficient of diffusion of pure silicon forthese substances. As a result, the effect of the insertion plate 4 as adiffusion barrier is ensured.

The covering plate 4 is advantageously heat resistant, at least up to amelting temperature of silicon, in particular to at least 1450° C.

The insertion plate 4 has less than 1%, in particular less than 0.1%, inparticular less than 0.01%, impurities.

The insertion plate 4 may have a coating comprising at least onesubstance from the quaternary system Si—C—O—N. Possible coatings are, inparticular, silicon carbide (SiC), silicon nitride (Si₃N₄) and silicondioxide (SiO₂). Furthermore, the coating may consist of B-Nmodifications or have modifications of this type. It may also havemixtures of Si—C—O—N and B—N or consist of mixtures of this type.

The insertion plate 4 has a thickness in the range from 1 μm to 10 mm,in particular in the range from 50 μm to 5 mm, in particular in therange from 0.4 mm to 3 mm, in particular in the range to 1 mm.

The thinner the insertion plate 4, the less the heat flow from thesilicon melt 3 is influenced by it. The thicker the insertion plate 4,the more reliably it can prevent a diffusion of impurities from thewalls 5, 6 of the container 2 into the silicon melt 3. The given rangeshave proven to be an optimal compromise to fulfill these two mutuallyconflicting requirements.

In order to effectively prevent the diffusion of impurities from thecontainer 2, in particular the base wall 5 thereof, the insertion plate4 has a low porosity. It is in particular closed-pore in configuration.It preferably has a density of at least 90% by volume, in particular atleast 95% by volume. High density values of this type cannot be achievedby coatings, which are generally applied from a dispersion onto theinside of a crucible, and which have a high porosity.

The material of the insertion plate 4 preferably has a thermalcoefficient of expansion, which differs by at most 10%, in particular atmost 5%, in particular at most 1%, from that of the material of the basewall 5 of the container 2.

To produce the device 1 for producing silicon blocks 30, the insertionplate 4 is firstly provided. For this purpose, silicon dioxide can, inparticular, be melted and solidified to form a block of a suitable size.The insertion plate 4 can then be cut from this block. The container 2is then provided and the insertion plate 4 is arranged therein.

In different embodiments of the invention it may be provided that theinsertion plate 4 is provided with a coating before being arranged inthe container 2 or after being arranged in the container 2. For detailsof the coating, reference is made to the above description. The coatingmay have different functions. It may, on the one hand, facilitate theremoval of the silicon block from the container 2. It may also influencethe crystallization of the silicon melt 3 in the container 2. Fordetails see DE 10 2005 028 435 A1, DE 10 2005 029 039 A1 and DE 10 2005032 789 A1.

Moreover, sapphire can be used as the nucleus for monocrystalline orcoarse-grain silicon.

When applying the coating after arranging the insertion plate 4 in thecontainer 2, the coating can also be used to seal remaining intermediatespaces between the insertion plate 4 and the side walls 6 of thecontainer 2 and/or with a multi-part configuration of the insertionplate 4, to seal between the individual components thereof

The coating in particular has a thickness in the range of 0.05 mm to 0.5mm.

To produce silicon blocks 30, in particular for photovoltaicapplications, the device 1, in particular the container 2, is firstlyprovided with the insertion plate 4 and the insertion plate 4 isarranged in the container 2. The silicon melt 3 is then provided in thecontainer 2. This may take place by pouring already molten silicon intothe container 2 or by melting silicon in the container 2. The siliconmelt 3 in the container 2 is separated by the insertion plate 4 from thebase wall 5 of the container 2.

The silicon melt 3 in the container 2 is then solidified by suitablecontrol of the heating/cooling elements 10, 11, 12. For details in thisregard, reference is made, for example, to DE 10 2005 013 410 A1.

A further embodiment of the invention will be described below withreference to FIG. 3. Identical parts have the same reference numerals asin the embodiment according to FIG. 2, to the description of whichreference is hereby made.

In this embodiment, a large number of insertion plates 4 are provided.These are configured in such a way that they cover the base 5substantially without gaps. They may, in particular, form a tiling ofthe base wall 5. Moreover, there is also provision in this embodiment tocover the region of the side wall 6 adjoining the base wall 5 withinsertion plates 4.

The insertion plates 4 to cover the side wall 6 are preferably orientedparallel to the latter. A separating joint 18 can remain between theinsertion plates 4 arranged on the base wall 5 of the container 2 andthe insertion plates 4 arranged on the side wall 6 of the container 2.The separating joint 18 has a free width of at most 5 mm, in particularat most 3 mm, in particular at most 1 mm.

To close the separating joint 18, a coating 19 may be provided, inparticular in the region of the separating joint 18. Reference is madeto the above description of the coating of the insertion plate 4 and/orthe container 2 for details of the coating 19. The coating 19 forms aprotection against the silicon melt 3 running behind the insertionplates 4.

In this embodiment, there is also provision to provide the side wall 6,at least in regions, with the coating 19. The side wall 6 is inparticular provided in the region not covered by the insertion plates,in other words in the region above the insertion plates 4, with thecoating 19. The coating 19 reaches, in particular, at least to a height,which is greater than a maximum filling height h_(max) to be expected ofthe silicon melt 3 in the container 2.

A further embodiment of the invention will be described below withreference to the figures. Identical parts have the same referencenumerals as in the above-described embodiments, to the description ofwhich reference is hereby made.

In the embodiment shown in FIG. 4, it is provided that a coating 20 isarranged between the walls 5, 6 of the container 2 and the insertionplates 4.

The coating 20 is made of a material that is difficult to sinter, forexample silicon nitride (Si₃N₄) or silicon oxynitride (Si—O—N).

The coating 20 is applied to the walls 5, 6 of the container 2 from adispersion, in particular a powder, with particles, the diameter ofwhich is in the range from 0.1 μm to 10 μm.

It is provided in this embodiment that the insertion plate 4 isconfigured in the region of the side wall 6 in such a way that,proceeding from the base wall 5, perpendicular thereto, it has anoverall extent, which is greater than the maximum filling height h_(max)to be expected of the silicon melt 3 in the container 2.

A further embodiment of the invention will be described below withreference to FIG. 5. Identical parts have the same reference numerals asin the embodiments described above, to the description of whichreference is hereby made. The container 2 is also provided in thisembodiment with a coating 20 in the region of the side walls 6 and inthe region of the base wall 5. An insertion plate 4 is in turn providedas the diffusion barrier. It is also provided in this embodiment that alarge number of crystal nuclei 21 are to be arranged on the insertionplate 4. The crystal nuclei 21 are, in particular, arranged parallel tothe base wall 5 of the container 2. They may, in particular, betriangular, rectangular, in particular square or hexagonal. They are, inparticular, configured in such a way that the base wall 5 of thecontainer 2 is covered by them in a tileable manner, i.e. substantiallywithout gaps.

The crystal nuclei 21 are separated from the base wall 5 by theinsertion plate 4.

It may be provided that a gap 22 is to be left between the crystalnuclei 21 and the side walls 6 of the container 2. The gap 22 may havean extent in the direction perpendicular to the side wall 6 in the rangeof 100 μm to 30 mm.

In principle, a single crystal nucleus 21 may also be provided. Apartfrom the possible gap 22, this may preferably substantially havedimensions such that the base wall 5 can be completely covered thereby.

The device according to FIG. 5 is advantageous, in particular forproducing monocrystalline silicon blocks 30. Care is taken here by meansof a suitable temperature control in the crucible that the temperaturein the region of the diffusion barrier is below the temperature, atwhich a eutectic forms between the material of the diffusion barrier andsilicon. The liquid silicon melt 3 preferably does not come into directcontact with the diffusion barrier. It is, in particular, separated bythe crystal nuclei 21 from the diffusion barrier.

In a further embodiment, the insertion plate 4 may also itself be usedas a crystal nucleus 21. An insertion plate 4 made of sapphire, forexample, can be used directly as a crystal nucleus 21 formonocrystalline or coarse-grain silicon.

FIG. 6 a schematically shows a service life topogram of a column 23,which has been cut out of a silicon block produced not according to theinvention. Also clearly discernable is a base region 24, also simplycalled the base, at a first end 25 of the column 23, an upper regioncalled a cap 26 on a second end 27 opposing the first end 25 and anintermediate region 28. The base region 24 has an extent of more than 60millimeters in a longitudinal direction 29.

The regions 24 and 26 are defined by the averaged charge carrier servicelife therein. The region, in particular, of the silicon block 30, whichextends from the first end 25 in the longitudinal direction 29 and inwhich the charge carrier service life averaged laterally, i.e.perpendicular to the longitudinal direction 29, is less than apredetermined limit value of a maximum of 3 μs, in particular a maximumof 2 μs, is called the base region 24. Accordingly, the averaged chargecarrier service life in the cap 26, i.e. in the region adjoining thesecond end 27 of the silicon block 30 in the longitudinal direction 29,is less than this limit value.

The course of the laterally averaged service life of the charge carrierof the column 23 shown in FIG. 6 a is shown in FIG. 6 b. While theaveraged service life in the region of the base 24 and the cap 26 issubstantially less than 2 μs, the laterally averaged service life of thefree charge carrier in the intermediate region 28 is up to 8 μs. It is,in particular, substantially over the total intermediate region 28, i.e.in every desired height of the intermediate region 28, at least 2 μs, inparticular a least 3 μs, in particular a least 4 μs, in particular atleast 5 μs, preferably at least 6 μs.

As the base region 24 and the cap 26 are classified as less useful andtherefore discarded because of the low service life of the chargecarrier for further processing in photovoltaic applications, the yieldof the silicon block produced by a conventional method is at most about62%.

A comparison of the charge carrier service life is shown by way ofexample in FIGS. 7 and 8 by vertical sections through multicrystallinesilicon blocks 30. The silicon block 30 shown in FIG. 7 was producedhere without a diffusion barrier according to the invention. A siliconcarbide (SiC) wafer, which was placed on the base wall 5 of thecontainer 2 before the introduction of the silicon melt 3 into thecontainer 2, was used as an insertion plate 4 in the silicon block 30shown in FIG. 8. The SiC wafer is shown for illustration in FIG. 8.

As in FIG. 6 a, the base region 24, the cap 26 and the intermediateregion 28 can in turn be clearly discerned in the silicon block 30 shownin FIG. 7. Moreover, a central region 31 and a peripheral region 32surrounding it can be distinguished in the lateral direction, i.e. inthe direction perpendicular to the longitudinal direction 29. Takentogether, the section region between the intermediate region 28 and thecentral region 31 can be combined to form a core region 33 and theremainder of the silicon block 30 can be combined to form a peripheralregion 34. Like the regions 24 and 26 in the longitudinal direction 29,the peripheral region 32 is defined by the average charge carrierservice life. The averaged charge carrier service life in the peripheralregion 32 is less than a predetermined limit value of a maximum of 3 μs,in particular a maximum of 2 μs.

As can be clearly seen from FIG. 8, the arrangement of the insertionplate 4 on the base wall 5 of the container 2 leads to a substantialreduction in the extent of the base region 24 in the longitudinaldirection 29.

The core region 33 is also defined by its charge carrier service life.In the silicon block 30 produced according to the invention, i.e. whenthe insertion plate 4 is arranged on the base wall 5 of the container 2,the laterally averaged charge carrier service life in the core region 33at each height is more than a predetermined minimum value of at least 2μs, in particular at least 3 μs, in particular at least 4 μs, inparticular at least 5 μs. As can clearly be seen from FIG. 8, thearrangement of the insertion plate 4 on the base wall 5 of the container2 leads to a clear reduction in the extent of the base region 24 in thelongitudinal direction 29 and therefore to an increase in the coreregion 33. In the embodiment shown, the base region 24 in thelongitudinal direction 29 has an extent h of less than 1 cm. In general,the base region 24 of the silicon blocks 30 produced according to theinvention in the longitudinal direction 29 proceeding from the first end25 has an extent of at most 50 mm, in particular at most 30 mm, inparticular at most 10 mm.

Accordingly, the arrangement of insertion plates 4 in the region of theside walls 6 leads to a reduction in the extent of the peripheral region32 in the direction perpendicular to the longitudinal direction 29. Theperipheral region 32, in the lateral direction, i.e. in the directionperpendicular to the longitudinal direction 29, has a thickness of atmost 50 mm, in particular of at most 30 mm, of at most 10 mm.

It can, in particular, be achieved by the diffusion barrier that theedge region 34 has a maximum thickness of at most 50 mm, in particularat most 30 mm, in particular at most 10 mm, in particular at most 5 mm,in particular at most 3 mm, in particular at most 1 mm.

The yield can therefore be substantially improved by the methodaccording to the invention. A yield of at least 70%, in particular atleast 75%, in particular at least 80%, can be achieved, in particular,by the method according to the invention.

FIGS. 9 and 10, by way of example, accordingly show, as in FIGS. 7 and8, the charge carrier service life with a vertical section throughmonocrystalline silicon blocks 30, which have been achieved with the aidof a template of crystal nuclei 21. FIG. 9 shows a silicon block 30here, which was produced without the arrangement of a diffusion barrierin the crucible. In the example shown in FIG. 10, an SiC wafer on thebase wall 5 was used as the insertion plate 4, which, as was shownlater, slid into the region of the lefthand side wall 6 of the container2 in FIG. 10. The clear influence of the insertion plate 4 on the chargecarrier service life in the silicon block 30 can in turn be seen fromthe figures.

Obviously, the details of the individual embodiments, in particular theone-part or multi-part configuration of the insertion plate 4, thearrangement of insertion plates 4 in the region of the side wall 6 andthe provision of coatings 19, 20 can be combined with one another asrequired.

In principle, it is also conceivable, in order to produce the insertionplate 4, to coat a carrier with material, which is used as a diffusionbarrier for impurities in the container 2. For details of a material ofthis type, reference is made to the description of the insertion plate4.

While the insertion plate 4 in the embodiments according to FIGS. 2 to 4is mechanically rigid, it may be advantageous to make the diffusionbarrier flexible, i.e. as an insertion foil. The rigid insertion plate 4and the flexible diffusion barrier are collectively called the coveringelement. The covering element comprises, in particular, the insertionplate 4 or the insertion foil.

The covering element in general leads to a separation of the siliconmelt 3 from the base wall 5 of the container 2. It prevents thediffusion of impurities from the container 2 into the silicon melt 3and/or the silicon block 30 to be produced.

A separation of this type to prevent the diffusion of impurities canalso be achieved in that the insertion plate 4 is arranged separated byspacers 35 from the base wall 5 of the container 2 and arranged thereon.According to a further embodiment, the covering element is, in otherwords, formed by an insertion plate a, which is separated by spacers 35from the base wall 5 of the container 2. A nucleus template or aplurality of nucleus templates, for example one or more seed crystals,in particular made of monocrystalline silicon, is used as the insertionplate 4a here. The nucleus templates are, in particular arrangedabutting, in other words touching one another in a direction transverseto the base wall 5. They may also form a dense covering of the base wall5.

The insertion plate 4a may also be configured in accordance with theabove-described embodiments, or comprise a correspondingly configuredinsertion plate 4, in particular in addition to the nucleus templates.

In the above embodiments, the container 2 is preferably provided with acoating 20.

Instead of the silicon melt 3, a melt made of another material canobviously also be solidified by means of the device 1 according to theinvention. This may, in particular, be non-ferrous metal melts, inparticular with a fraction of silicon or germanium.

1. A device (1) for producing silicon blocks (30) for photovoltaicapplications, comprising a. a container (2) for receiving a silicon melt(3) with i. a base wall (5) and ii. at least one side wall (6), b. meansfor reducing the diffusion of impurities from at least one of the walls(5, 6) of the container (2) into the silicon melt (3) and the siliconblock (30), c. wherein the means for reducing the diffusion ofimpurities comprise at least one covering element (4; 4 a) to at leastpartially cover at least one of the walls (5, 6) of the container (1).2. A device (1) according to claim 1, wherein the at least one coveringelement is configured as an insertion element for insertion into thecontainer (2), which insertion element comprises an insertion plate (4;4 a).
 3. A device (1) according to claim 1, wherein the at least onecovering element is configured as one of the group of an insertion plate(4; 4 a) and an insertion foil.
 4. A device (1) according to claim 1,wherein the covering element (4; 4 a) is configured in such a way thatat least the base wall (5) of the container (2) can be at least 90%covered by the covering element (4; 4 a).
 5. A device (1) according toclaim 1, wherein the covering element (4) is configured in such a waythat at least the side wall (5) of the container (2) can be at least 90%covered by the covering element (4).
 6. A device (1) according to claim1, wherein the covering element (4; 4 a) is at least partially made of amaterial, which has a melting point which lies above the melting pointof silicon.
 7. A device (1) according to claim 1, wherein the coveringelement (4; 4 a) is at least partially made of a material, which has adiffusion constant in relation to silicon, which, at temperatures in therange of 20° C. to 1500° C., is at least 0.5 times as great as thediffusion constant of one of the transition metals titanium, vanadium,chromium, manganese, iron, cobalt and nickel in relation to silicon atthe corresponding temperatures.
 8. A device (1) according to claim 1,wherein the covering element (4; 4 a) is at least partially made of amaterial, which has a diffusion constant in relation to silicon, which,at temperatures in the range of 20° C. to 1500° C., is at most 0.3 timesas great as the diffusion constant of one of the transition metalstitanium, vanadium, chromium, manganese, iron, cobalt and nickel inrelation to silicon at the corresponding temperatures.
 9. A device (1)according to claim 1, wherein the covering element (4; 4 a) is at leastpartially made of a material, which has a diffusion constant in relationto silicon, which, at temperatures in the range of 20° C. to 1500° C.,is at most 0.2 times as great as the diffusion constant of one of thetransition metals titanium, vanadium, chromium, manganese, iron, cobaltand nickel in relation to silicon at the corresponding temperatures. 10.A device according to claim 7, wherein the covering element issimultaneously a nucleus template for the crystallization of the siliconmelt.
 11. A device (1) according to claim 1, wherein the coveringelement (4; 4 a) has at least one fraction selected from the group ofthe refractory metals, the compounds thereof as well as silicon dioxide(SiO₂), silicon carbide (SiC) and silicon nitride (Si₃N₄) and thecompounds thereof, as well as aluminum oxide (Al₂O₃).
 12. A device (1)according to claim 1, wherein the covering element (4) has a density ofat least 90% by volume.
 13. A device (1) according to claim 1, whereinthe covering element (4; 4 a) is produced from an amorphous silicondioxide (SiO₂).
 14. A device (1) according to claim 1, wherein thecovering element (4; 4 a) has a thickness in the range from 0.001 mm to10 mm.
 15. A device (1) according to claim 1, wherein the coveringelement (4; 4 a) has a thickness in the range from 0.05 mm to 5 mm. 16.A device (1) according to claim 1, wherein the covering element (4; 4 a)has a thickness in the range from 0.4 mm to 1 mm.
 17. A device (1)according to claim 1, wherein the covering element (4; 4 a) has acoating comprising at least one substance from at least one of thequaternary system Si—C—O—N and the binary system B—N.
 18. A device (1)according to claim 1, wherein the covering element (4; 4 a) has acoating of one of the group of silicon nitride (Si₃N₄) and boron nitride(BN).
 19. A device (1) according to claim 17, wherein one of the groupof one nucleus template and more nucleus templates are used as thecovering element (4 a).
 20. A device (1) according to claim 17, whereinthe at least one nucleus template is made of monocrystalline silicon.21. A method for producing silicon blocks (30), comprising the followingsteps: providing a container (2) to receive a silicon melt (3)comprising a base wall (5) and at least one side wall (6), providing andarranging a means for separating the silicon melt (3) in the container(2) from the base wall (5) thereof, providing a silicon melt (3) in thecontainer (2), solidifying the silicon melt (3).
 22. A method accordingto claim 21, wherein the means for separating the silicon melt (3) inthe container (2) from the base wall (5) thereof comprises an insertionelement (4; 4 a), by means of which the base wall (5) is at least 50%covered, wherein the insertion element (4; 4 a) prevents the diffusionof impurities from the base wall (5) of the container into the silicon.23. A silicon block (30) produced by solidifying a silicon melt (3)using a device (1) for producing silicon blocks (30) for photovoltaicapplications, comprising a. a container (2) for receiving a silicon melt(3) with i. a base wall (5) and ii. at least one side wall (6), b. meansfor reducing the diffusion of impurities from at least one of the walls(5, 6) of the container (2) into the silicon melt (3) and the siliconblock (30), c. wherein the means for reducing the diffusion ofimpurities comprise at least one covering element (4; 4 a) to at leastpartially cover at least one of the walls (5, 6) of the container (1),wherein one of the group of one nucleus template and more nucleustemplates are used as the covering element (4 a), wherein the siliconblock (30) has a core region (33) extending in a longitudinal direction(29) and an edge region (34) adjoining the latter with a charge carrierservice life that is reduced compared to the core region, wherein thecharge carrier service life averaged over the core region (33) is atleast 2 us, and wherein areas of the edge region (34) adjoining theinsertion plate (4) in the longitudinal direction (29) have a smallerextent in the longitudinal direction (29) than non-adjoining areas ofthe edge region (34).